An induction motor is driven by currents generated by voltages applied to the phases of a stator of the induction motor, which are in general AC currents of different frequency. This frequency starts from zero (or close to zero) when the motor is standing and increases according with a speed command (e.g. in a vehicle with an operator aboard, a throttle actuated by the operator supplies the speed command).
Throughout the operative mode of the control the phases of the induction motor are subject to the AC currents. These currents may have different values (depending on the load and the dynamic torque), but they are always present because at least the magnetisation current generated by the main inductance is always drained by the motor including the case of a null output torque (synchronism).
Unlike a Brushless Permanent Magnet motor, the control of an Induction motor does not ask for the rotor position; it only asks for the rotor speed. In an induction motor, the frequency must be tuned in a narrow frequency band, around the angular motor speed in order the control properly works. The correct terminology is that the ωslip of the motor, i.e. the difference between the frequency applied to the motor phases and the electric angular rotor speed (electric angular rotor frequency) must be limited in the linear torque characteristic of the induction motor (FIG. 1). When the ωslip is positive, the motor is driving the load; when the slip is negative, the motor is braking the load.
When the ωslip is outside the linear torque characteristic, the torque collapses although the inverter is supplying the maximum current to the motor (Imax Clamped area in FIG. 1). In order to verify if the control is properly working or not it is just enough to know the ωslip. As the applied frequency is generated by the control, the angular speed is the only unknown variable.
The prior art deals with methods to measure the speed and position of the rotor in an electrical motor without speed sensors at the motor shaft (sensorless control). These methods normally work by introducing proper test signals, together with the main controlling quantities, in the motor to show up some saliency or nonlinear property of the rotor self. Then by monitoring the effect of the test signals injection, it is possible to detect the speed and the position of the rotor (e.g. the INFORM method could be a good example for this prior art method; INFORM=INdirect Flux detection by On-line Reactance Measurement).
Unfortunately these methods are quite complicated and can fail whenever the saliency (degree of alignment in a preferred direction of the magnetisation) of the magnetic core is not high enough.
At last, all of these methods with signals injection, generate harmonics in the motor with audible noise and torque ripple.
Anyway, applications are known of these methods in combination with Brushless Permanent Magnet Motor; for the Induction motor the application of these method is even more compromised because the saliency must be extrapolated from the main magnetic field. Meanwhile in a Brushless Permanent Magnet Motor, the main magnetic field at the rotor has a fixed value in time and rotor coordinates (due to the Permanent Magnets), in an Induction motor the main magnetic field has a complicate evolution due to the wide dynamic profile (different levels and slopes) of the magnetisation current.